This invention relates to a turbine blade and to a turbine incorporating the blade. While the invention is primarily concerned with steam turbines it is also applicable to other turbines and to compressors. The term "turbine" is used in this specification to include machines of this kind having aerofoil blades. It is also primarily concerned with fixed blades in turbines but is not exclusive to them.
Turbine efficiency is of great importance, particularly in large installations where a fractional increase in efficiency can produce very large cost savings. A considerable amount of money and effort is continually expended therefore on research into the blade design, this being a critical component.
For many years the conventional blade has been of aerofoil cross-section, the (fixed) blade extending radially between inner and outer end blocks, and the blade being of prismatic form, i.e. generated by a line moving parallel to itself and intersecting an aerofoil section. The orientations of both fixed and moving blades about their respective blade axes has also been standardised for this prismatic blade design, this orientation being defined by the blade stagger angle between the turbine axial direction and a line tangential to the blade leading edge and trailing edge circles on the pressure face of the aerofoil blade.
A known improvement in the performance of the prismatic blade in the turbine is achieved by imposing a `lean` on the blade, i.e. tilting it about its root in a circumferential plane i.e. one transverse, or perpendicular, to the turbine axis. This `lean` produces a variation in the mass flow at outlet of the blade from the root to the tip. The radially inner and outer ends of the blade are referred to as the root and the tip despite the fact that both root and `tip` are terminated by the end walls of the supporting rings 21 and 22 shown in the accompanying FIG. 1.
Since the circumferential spacing of the blades (ie pitch) increases progressively from the root to tip, the position where the throat line intersects the suction surface moves upstream with increased radius. Owing to the convex curvature of the suction surface this leads to an increase in the outlet angle from about 13.degree. at the root (relative to the tangential direction) to about 15.degree. at the tip. This is illustrated in the accompanying FIG. 6.
The blade outlet angle .alpha. is illustrated in FIGS. 3(a) and 3(b) of the accompanying drawings and is defined as sin.sup.-1 (throat/blade pitch).
From the same figures the following parameters appear. The throat is the shortest width in the blade passages. It normally extends from the pressure surface of a blade at the trailing edge and is orthogonal to the suction surface of the adjacent blade.
The stagger angle is the angle between the axis of the turbine and the tangent line touching the leading and trailing circles of the aerofoil section.
The blade chord length is the overall extent of the blade along the stagger angle tangent line.
Modifications to the basic prismatic blade design have in the past been proposed. For example, in the Hitachi Review Vol 27, No. 3 of 1978, twisted and other blade forms were proposed. In what was referred to as the `controlled vortex nozzle design` there was described a nozzle (i.e. fixed blade) which conformed to the conventional prismatic blade form for the lower half of its radial height but which had a progressively finer setting angle for the upper half. The setting angle is the angle by which the aerofoil section at any blade height is rotated within its own plane from the normal disposition for a prismatic blade. A fine setting indicates a rotation of the aerofoil section to reduce the throat and thus reduce the outlet angle and a coarse setting a rotation to increase it. FIG. 3 of this earlier article illustrates a continuous rotation of the blade section from the root to the tip, the setting angle becoming finer with increased blade height.
Despite the fairly comprehensive analysis of blade design and setting angle of this earlier study it has been found that none of the designs investigated achieve the degree of improvement that the present invention provides.
Thus, it is an object of the present invention to provide a blade design which provides a significant improvement in performance over previously known designs.